The present invention relates to electric switches of the type wherein at least one electrically conductive electrode is movable into and out of contact with another electrode to close or open an electric circuit.
Examples of such switches include tilt switches, reed switches, microswitches, and other mechanically, magnetically or electromagnetically driven switches.
A tilt switch is used for indicating the presence of an angular orientation through the creation of an electrical signal. Such uses range from thermostat controls and motion detectors, to ordinance devices and liquid level controls, among others. When the switch tilts, an electrically conductive medium moves along one fixed electrode and into contact with another fixed electrode to act as a shorting element. Liquid mercury provides an ideal conductive medium in such a case, because mercury can selectively wet certain areas, i.e., the electrode surfaces, without wetting the insulative surfaces. However, mercury possesses substantial drawbacks such as environmental pollution and toxicity. It is thus desirable to provide a non-mercury alternative to the mercury switch.
Workers attempting to satisfy that need have devised switches comprised of a casing forming a chamber containing a gaseous environment (e.g., argon) in which there is disposed a mobile conductive element, e.g., a gold plated ball, adopting the role of mercury. Strategically disposed within the chamber are fixed electrodes, i.e., fixed with respect to the casing. The gold plated ball functions as a movable electrode and as an alternative to the free flowing mercury. Thus, when the ball simultaneously contacts the fixed electrodes, an electrical signal is transferred. Those devices, however, suffer from low current carrying or switching capacity, high contact resistance, short life and/or electrical bounce.
The low current carrying capacity and high contact resistance occur even though the ball and fixed electrodes are in physical contact with one another, due to a shortage of contact area as a result of the irregular (non-smooth) surfaces of the electrodes. That is, even though electrodes may appear to be smooth, they exhibit microscopic irregularities, defining peaks and valleys. Actual contact between the surfaces occurs at opposing peaks, and may be insufficient to achieve a desired current flow especially if the non-contacting portions of the electrodes are spaced apart so far that electric current cannot bridge the gap.
Such actual contact between the electrodes will occur if the electrode surfaces are formed of a material (e.g., noble metals) which are inert with respect to oxygen. In the case of metals which react with oxygen, however, a thin insulative oxide film may form on the electrode surfaces, whereby current flows across the peaks by way of a quantum tunneling effect, rather than as a result of direct contact between the conductive metals, thereby further inhibiting the current flow.
The other shortcoming, i.e., bounce, can occur when the ball impacts against a fixed electrode, or as vibrations occur after contact is made. The impact or vibrations can cause the ball to move away from the fixed electrode, thereby intermittently opening and closing the circuit.
It would be desirable to provide a switch, especially a tilt switch, which minimize or obviates those shortcomings.